Photodetection circuits are used in a wide variety of applications. One very common application is the detection of movement of an object relative to some reference. In such systems, a mask and perhaps other components such as a scale are interposed between an LED or other light source and a photosensor such as a phototransistor. The mask or shutter, which may be a rotating disk, typically includes alternating opaque and transparent segments and is driven by the movement of the object being tracked. The rotation of the disk causes the light from the LED to alternately reach and be blocked from the photodetector. The movement of the object is thus converted to a crudely digitized signal from the photodetector.
To create a better defined digital signal, such as is required for use with the microprocessors used in some devices, the output of the photodetector is typically fed to a logic gate which exhibits some hysteresis. This is most commonly accomplished by using a gate with a Schmitt trigger input and a threshold voltage V.sub.th.
In many applications, the power drain of an LED/phototransistor pair can be substantial, so that power reductions are desirable. One application where power reductions are useful is an electronic mouse. One technique for reducing the power consumed by the optoelectronic device is to pulse the LED selectively.
While pulsing the LED helps to reduce power consumption, it also exacerbates side effects which have generally been considered undesirable in photodetector circuits. Significant among these is the effect of parasitic capacitances in the photodetector, which is typically a phototransistor. The effects of these parasitic capacitances appear in the dynamic mode of the transistor, and increase in significance when the LED is pulsed. In most prior art circuits using phototransistors, the circuits are designed to minimize the parasitic capacitance, including the addition of circuitry to counteract its effects.
The most important parasitic capacitance is typically the parasitic base/collector capacitance, C.sub.bc. The parasitic effects have generally been of even greater concern in pulsed LED systems. In at least some instances additional hardware has been used to compensate for the effects of these capacitances.
As explained in greater detail hereinafter, some methods for reducing power involve reducing the current required by the phototransistor by increasing the value of the load resistor. However, because of the effects of the parasitic capacitances, such increase in load can result in unacceptable inccreases in response time. Thus, the amount of power reduction which can be obtained by such methods is limited, and in at least some cases unacceptable.
One device in which such power reduction methods yield unacceptable results is the electronic mouse. One form of electronic mouse uses a plurality LED/phototransistor pairs to convert X-Y motion to a digital representation of distance. Typically, four LED/phototransistor pairs with their respective shutters are used to provide an accurate representation of the magnitude and direction of movement in the X-Y plane. The accuracy of the digital conversion is determined by the number of pulses generated per unit of distance traveled. For correct movement calculations, at least 4,000 samples per second (or about one every 250 microseconds) are needed when the mouse is moved at reasonable speeds. At these sampling rates, new methods are required for power reduction.